For the case of high dose, low energy implants, the potential advantage of using molecular ions containing multiple atoms of elements of interest has been well recognized for several years. For a given ion beam current the dose is increased in proportion to the atomic multiplicity of the element of interest. Such ions can be extracted from a source and transported to the wafer or other target substrate at a much higher energy, in proportion to the ratio of the molecular mass to the atomic mass of the element of interest. Consequently, for relatively high dose implants, the wafer throughput is not as seriously limited by the internal space charge forces and the intrinsic thermal ion temperature within the ion beam. Also, for a given dose, the electrical charge delivered to the wafer by the beam is substantially less.
However, it is desirable to overcome a number of drawbacks that exist when an attempt is made to use such molecular ions in a conventional implanter. Firstly, the ion source of a conventional ion implanter has a relatively high density, hot plasma and heavy molecular ions are substantially disintegrated by such a source, often resulting in a low molecular ion yield. Secondly, the molecular ions are often generated with a range of masses as a result of various amounts of hydrogen atoms within the ion and also as a result of the binomial distribution of isotopic masses if there is more than one isotope of an element present in the generated ions. The different mass ions generally describe different paths through the implanter beam line and as a result can produce undesirable angular and/or dose variations over the surface of a wafer. Thirdly, the relatively high mass of the molecular ions limits the single atom implant energy, often to just a few keV because of the limited field strength and size of the conventional analyzer magnet (and other magnetic elements if used).
To minimize the commercial costs associated with constructing and operating an ion implanter tool, it is also desired to have an ion implanter that is multipurpose, capable not only of overcoming the drawbacks associated with implanting the molecular ions, but also capable of implanting common monatomic dopant species.
Furthermore, it is desirable, even with ion implanters that are constructed principally for implanting common monatomic dopant species, to enable efficient operation over a wide range of ion densities in the beam in order to meet the large dynamic range of doses generally required, to provide high ion purity at the target with respect to the ion energy resolution and with respect to freedom from ion species which would degrade the semiconductor structures, and to impinge ions on to the wafer substrate with a small angular spread, good angular definition, and good dose uniformity over the surface of the wafer.